Filter for fluid flow systems



Sept. 3,' 1946. w. o. TAIT FILTER FOR FLUID FLOW SYSTEMS Filed nine 50, 1944 /fv aff/WOR M4 0. 7A? T Patented Sept. 3, '1946 FIIAQTi-t roaFLUm FLOW SYSTEMS i `Williamp.Tait,"Garden City, N. Y., assignor to Sperry Gyrscop Company, Inc., a corpora- "tion of New York [1f Y Applicationniiisd1944,seria1N0.54as55 'A i (Banner (Cl. 21a-'7164) 'Ihis invention relates to the artV of filtering and is particularly concerned with a novel method of and apparatus for filtering fluids in reverse flow circulating systems, While theY invention hasa special utility inrconnection with high pressure hydraulic servo systems it has awide range of application in filtering fluids lin. fluid circulating systemsof all types. 'A A, u Y A For several reasons filtration of the working fluid in reversible ow high pressure systems has not heretofore appeared feasible.- With the filtration methods previously tried, reversal of fluid flow in thesystem has meant a reversal of pressure within the filtering unit accompanied by the return of foreign particles, and other previously removed substances to the main circulation chan-- nels of the system, a result highly undesirable. A similar result is produced in unifiow systems when excessive loading causes a temporary Vreversalof flow, of the working fluid. Furthermore in high pressure systems a filtering element strong enough to resist system pressure frequently allows small but destructive foreign lparticles to 'pass through the filterV and lcontinueV in circulation. There has also been the further diiiicult problem of obtaining a filtering rate high enough to effece tively cleanse the fluid Without unduly obstructing the flow of fluid through thevsysterm.4 In hydraulic systems used for control purposes, particularly if high pressures are used, any compressibility of the working medium introduces error into the operation of the controls. Obviously adding a filter to the system usually entails increasing thefiuid content of the system, thus increasing the compressibility error above mentioned. f

The novel apparatus and methods of the present invention overcome the above cited difficulties in the following ingenious manner. By introducing a constriction into the path of the main fluid stream, an'increase in velocity and a reduction in the static pressure of the stream is created in the region of the constriction, the velocity of flow and the static pressure of the stream in regions removed from the constricted region being relatively little affected by, this change inthe shape' of the channel through which the fluid passes;V

There is thus created a pressure difference between points located in the reduced diameter `region and `points located in the full diameter regions of the flow channel which is a function of the rate of stream flow and which operates-to maintain the reduced diameter region at reduced static pressure irrespective of the *Y direction of fluid flow inthe constricted region ofthe flow channel. In other words, the pressure difference above mentioned is in the nature of a drop in static pressure between any section in the unconstricted region of the l fluid channel and the throat of the channel Whether that section be upstream or downstream of the throat. Hence, by operatively connecting a filter ,between any point subject `to relatively high pressure and a point of reduced pressure in the fiow channel, a filtering system is produced in which the pressure drop always occurs in the same direction irrespective of the direction of fluid flow in the other parts of the system, whereby a portion of the circulating fluid .passes undirectionally through the filter even when a reversing fluid flow exists in the main flow channels of the system.

Since the pressure drop between the unconstricted and constrictedregions of the fluid stream is independent of the actual working pressure ofthe system it is possible to control this pressure drop by judicious selection of the cross sectional area of the constricted region relative to the cross sectionalgarea of the unconstricted'section and thus keep the pressure drop across the filter small enough to prevent rupturing of the delicate filtering elements often used to remove small particles from the fluid in the system', even in installations where the working pressure is relatively high. t v

Moreover since the filtering element is located out of the main channel of flow and may be connected therewith-by short, low resistance conduits, filtering of the system working-medium is 'obtained With only a slight obstruction ofthe invention makes possible the filtering of the fluidA without materially increasing the vvolumetric capacity of the system, hence the effecten the compressibility error of the system is kept at a minimum. v

The objects of this invention may therefore be briefly enumerated -asfollowsz the provision of a filter and associated circulatory system suitable for use in a reversible flow fluid system; the provision of a filter which may be used withhigh pressure fluid systems without danger of rupture of relatively delicate filtering materials or fine mesh screens; the provision of a filter having an associated circulatory or auxiliary filtering circuit arranged for minimum obstruction of the working passages of the fluid flow system in which it is-used; the provision of filtering apparatus having the smallest fluid content consistent with effective filtering ofthe fluid in the system with which the iilter is employed; the provision of a filtering method suitable for use in relatively high pressure iluid flow systems; the provision of a ltering system which will induce constant recirculation of the fluid through the lter, thus producing high filtering rate and effective cleansing of the uid circulating through it; and the provision of filtering apparatus having no moving parts to wear rapidly or break loose and inter fere with the operation of the system in which the lter is installed.

Other objects and uses of the invention willr be apparent from the following description and the accompanying drawing wherein,

Fig. 1 is a longitudinal section through one form of filter incorporating the principles of this invention.

Fig. 2 is a sectional view on the line 2-2 of Fig. 1 taken at right angles to the main path of iiuid ow through the ydevice of the invention, the filtering element being shown partly in elevation and partly in section.

Fig. 3' is a longfi'tudinalV section of'- a slightly modied version of the filter of'Fi'g. 1, the filtering element being shown partly in section and partly in elevation.

The apparatus of' the invention, in general, comprises a unit adapted to be' inserted at a convement point in the main' path offl'ow of a closed circulatory system whose working substance or fluid may require filtering, this unit containing a passageway which permits the iiuid to pass through with minimum obstruction ofthe fluid stream. In addition, means are provided for diverting a portion ofthe fluid from the main channel of flow into an auxiliary or local path of flow wherein the ilui'd is forced through a suitable ltering element and after cleansing is returned to the main channel of flow at a point where the cross' section of this channel has been reduced for the purpose of creating a low pressure point in the main fluid stream. Thus, after filtering, the fluid in the auxiliary or filtering channel is returned to and intermingled withv the fluid moving through the main passageways of the control or operating system.

As illustrated in Fig; 1, one form of apparatus in which the principles of the invention may be embodied comprises a main channel or' body member having a channel 6 for passing' uid completely through body member 5y from supply pipe 1 to return pipe 8 of the system with which the' apparatus of the invention is employed, which pipes are secured for uidtight" connection to member 5 as by bolts 9 and flange- Iusing pack"- ing glands or sealing rings to prevent the escape of iiuid from the interior of the' system. Channel 6 is formed with .a full diameter section I2 'at' each end thereof and with a throated or constricted section I4 intermediate theA full diameter sections I2, typically with symmetrically tapered funnel-shaped passageways I5, on both sides thereof for connecting throat I4 to the full diameter or unconstricted sections I2, in the manner of a Venturi tube.

Channel member 5 also includes a hollow laterally projecting, externally threaded collar I6 upon which an internally threaded cap or cover plate I1 is adapted to be screwedso ,as to form, with the aid of sealing ring I8, a fluid tightv connection. 'Ihe chamber formed by collar I6 and cap II is appropriately shaped and properly sized to form a filtering chamber I9. Filtering chamber I9 may' contain a replaceable cartridge type filtering element 2| held centrally secured there- Cil in by boss 22 on cap I1 and boss 23 on channel member 5, filtering element 2l being designed to provide a large filtering surface in a unit of compact dimensions.

Channel member 5 contains at least two, and if desired may contain more than two, lateral openings or conduits into ltering chamber I9, these openings being as short as possible and being shaped to provide free and unobstructed passage for the iluid between the main flow channel 6 and the ltering chamber I9. One of these lateral openings 24 which connects filtering chamber I9 to throat I4 serves as an exit conduit or outlet from the filtering chamber .to the main channel of flow 6. One or more of these lateral openings 26 connects lteri'ngchamber I9 to a point of low velocity and high static pressure such as sections I2 or any relatively high pressure point located elsewhere in `the main channels of flow; such connection or connections serve as inlets to lead fluid from channel 6, to filtering chamber I9, where the iiuid ls cleansed by passing through ltering element 2|, thereafter flowing' back to channel 6 by way of lateral opening 24. This invention provides apparatuscharacterizedV by marked simplicity and highlyv eiective filtering performance which is obtained with a negligible increase in the resistance .to the ow of thev working substance through the passageways of thel system.

In operation, iiui'd under high staticA pressure enters channel 6 or'member 5- from' supply pipe 1, and ows, for example as shown by arrow F in Fig. l, from the' unconstricted section I2 at the right hand end of channel 6, through the tapered approach passageway I5 into throat I4, out of throat I4 through they tapered exit passageway I5 into the unconstricted section I2', at the left hand end of channel 6, from which region it then flows into-` return pipe 8. As the fluid passes from the upstream, unconstricted section I2 into the throat' I4, it experiences an increase in its velocity of flow' and a decrease in its static pressure; as it continuesV ori its way flowing back into the downstream, unconstricted section I2, the reverse phenomenon occurs, the velocity of stream flow decreases and the static pressure of the fluid increases until it is' again atf the original value, except for slight frictiona'l losses. It will be noted that the pressure differential thus created between the unconstricted regionsl of the channel of'flow and the constricted region I4 is alwaysv in the nature of a fall in static pressure; and this is true whetherv the unconstricted region I2 be upstream or downstream of throat I4'. The. laterall openings 26, being connected to regions of relatively high pressure I2, will always be inlet` passageways to filtering chamber I9, and lateral opening 24v being connected to the region of relatively low static pressure, i. e'., throat I4, will alwaysA be the outlet passageway for flltering'chamber I9, irrespective of whether the fluid is flowingY from right to left or vice versa in channel 6. A unidirectional now of uid through inlets' 26, iiltering' element 2|., and outlet 24 is thus providedA that isnot affected by' a reversal of ilow through channel 6; Hence any foreign particles removed from thef fluid stream by' lter 2| will not be'A discharged back into the working iiuid ofthe system by a reversal' ofV the direction of flow in the system. Moreover, since the magnitudel of` the pressure differential created between throat |-4 andthe full' di'- ameter regions '2 at the mouths of the' Venturi section in' channel 6, is a function of the velocity of flow at these points, i,he-ap`paratus of the in vention provides a convenient means for creating a safe pressure diierential across a selected filtering element irrespective of whether the system operating pressure is above or below the rupturing pressure of the particular filtering element. Filtering of minute particles from the working fluids in relatively high pressure systems thus becomes practicable through the use of this invention.

It will be readily discerned that locating the ltering chamber and filtering elementl below the Venturi section of the main flow channel results in two additional advantages. Firstly, any foreign particles which do not adhere to the filtering element will fall to the -bottom of the ltering chamber rather than pass back into the main fluid stream as they would do if the lter were above rather than below the main flow channel. Secondly, since air is lighter than the working medium ordinarily. used in such systems, any bubbles of air which may accumulate on the exterior of the filtering element are free to rise back into the main flow channel during periods when the fluid in this channel is motionless, thus providing a self-Venting filter. i

Tests of the above described ltering device have shown that a pressure difference as low as 20 lb. per square inch may be obtained across the filter in a system where the load pressure reaches as high as 2000 lbs. per square inch, and that particles with dimensions as small as 10 microns may be ltered out of the working duid by the apparatus of invention.

The version of the invention illustrated in Fig. 3 is `but slightly different from the device of Fig. 1. In the Fig. 3 version the taper of the approach and recession passageways of the Venturi section is more gradual and a cover plate 21 secured to channel member 5 by holding bolts 28, 29 is used in place of screw threaded cap l1 of Fig. l. Otherwise a slight difference in physical dimensions is the feature primarily distinguishing the Fig. 3 filter from the Fig. 1 filter. In use, these two versions of the invention exemplify the same operating principle.

`Since the above described inventive concept could be readily embodied in many apparently different physical forms without departing from the scope of this invention, it is intended that all matter contained in the specication or shown in the accompanying drawing be interpreted in an illustrative and not in a limiting sense.

Having thus illustrated the manner in which the principles of this invention may be applied, what is claimed is:

1. A filter for closed fluid flow systems comprising a body member arranged for fluidtight attachment to spaced conduit terminals of said system, a main flow channel passing through said body member, a symmetrically tapered Venturi section having equal angles of approach and recession centrally located in said flow channel, a hollow externally threaded collar integral with and projecting laterally from said body member,

i placeable filtering tion, thus providing a simple, rugged filter particularly suitable for use in relatively high pressure iluid flow systems.

2. A filtering device for filtering fluid comprising a conduit section adapted to form a Dart of a closed fluid-circulating system and shaped to provide a venturi responsive to flow in either direction to produce a reduced static pressure and yincreased velocity of the fluid at one point in said conduit, an auxiliary passage leading from another point in the same conduit as the first point but in a region of higher static pressure and discharging into the conduit at said one point, and a ltering element in said passage whereby a portion of the fluid flowing in the system is directed through the filtering element always in the same direction while flow in the system takes place in either direction.

3. A filtering device for filtering fluid comprising a conduit section adapted to form a part of a closed fluid-circulating system and shaped to provide a reduced-diameter throat and two oppositely directed and symmetrical tapered sections forming a venturi responsive to ilow in either direction to produce a reduced static pressure and increased velocity of the fluid at said throat, an auxiliary passage leading from a point in said conduit adjacent the outer end of one tapered section and discharging into the conduit at said throat, and a filtering element in said passage whereby a portion ofthe fluid flowing in the system is directed through the filtering element always in the same direction while ilow in the system takes place in either direction.

4. A filtering device for filtering fluid comprising a conduit section adapted to form a part of a closed fluid-circulating system and shaped to provide a Venturi responsive to flow in either direction to produce a reduced static pressure and increased velocity of the fluid at one point in said conduit, means forming a lter chamber at one side of the conduit section, an outlet from the chamber comprising a short direct passage to said one point, an inlet to the chamber comprising a passage leading from another point of higher static pressure in the conduit section, and a ltering element removably positioned in the chamber whereby a portion of the fluid flowing in the system is directed through the lterirng element always in the same direction while flow in the system takes place in either direction.

WILLIAM O. TAIT. 

